The COVID-19 pandemic significantly altered lifestyles and behaviors globally, including increased health consciousness and dietary supplement use. In Vietnam, there's a growing trend of consuming vitamins and dietary supplements, particularly foreign brands, to boost immunity. Facebook serves as a major platform for information dissemination and purchasing in Vietnam. This study addresses gaps in existing research by exploring the influence of User-Generated Content (UGC) on Facebook on Vietnamese consumers' purchase intentions regarding vitamins and dietary supplements during and after the COVID-19 pandemic. Specifically, it investigates the roles of UGC quality, quantity, trust, utilitarian value, and hedonic value in shaping this intention. The research questions are: (1) What are the key predictors of the heuristic-systematic model toward intention to use vitamin and dietary supplements during and post-pandemic?; (2) How does health-related information on social media affect individuals' behavioral intention toward supportive products? The theoretical framework integrates the theory of reasoned action (TRA), source credibility theory, and the Heuristic-Systematic Model (HSM) to understand the interplay between online information and consumer behavior.

The literature review examines the evolving definition and impact of User-Generated Content (UGC). While a universally accepted definition remains elusive, UGC is generally characterized by its public availability, creativity, and creation outside professional contexts. Numerous studies highlight the persuasive power of UGC, particularly regarding its influence on purchase intention across various platforms, including YouTube, dedicated apps, and Facebook. The Heuristic-Systematic Model (HSM), a dual-process model, is introduced as a framework for understanding how individuals process information from UGC, distinguishing between heuristic (less cognitive effort) and systematic (more in-depth) processing. Existing research on consumer purchase intention emphasizes the roles of perceived value (utilitarian and hedonic), trust, and eWOM. The review also notes the significant shift toward online shopping during the pandemic and a lack of research connecting UGC with dietary supplement purchases during this critical period. The literature review further examines the specific elements of UGC (hedonic and utilitarian value, information quality, and trust) and their relationships with purchase intention. Hedonic value relates to emotional benefits, while utilitarian value focuses on functional benefits. Information quality emphasizes objectivity, understandability, relevance, and sufficiency. Trust, defined as the willingness to depend on an exchange relationship, is highlighted as crucial for information acceptance and purchase decisions, particularly in the context of health-related products.

This cross-sectional study employed a convenient sampling method to collect data from 331 Vietnamese respondents via an online survey questionnaire using Google Forms. The questionnaire contained items measuring hedonic value, utilitarian value, information quality, trust, and purchase intention, adapted from previous studies and using a 5-point Likert scale. A pilot study was conducted with 20 Facebook users who had created content related to dietary supplements to assess content validity and refine the questionnaire. The final sample included 323 respondents who actively engage with Facebook for product reviews. Structural Equation Modeling (SEM) was utilized to analyze the data, employing a two-step approach with a measurement model and a structural model. The analysis examined the relationships between latent constructs, and mediation analysis using bootstrapping assessed the mediating role of trust.

The respondent profile revealed a majority aged 20-39, predominantly female, with income mostly between 5 and 15 million VND, and a high concentration in Southern Vietnam. Exploratory Factor Analysis (EFA) confirmed the multidimensionality of the constructs, and Confirmatory Factor Analysis (CFA) demonstrated good convergent and discriminant validity. SEM analysis showed that utilitarian value (β = 0.331, p < 0.001) and hedonic value (β = 0.142, p = 0.020) significantly and positively influenced consumer trust. Information quality (β = 0.418, p < 0.001), utilitarian value (β = 0.250, p < 0.001), and trust (β = 0.266, p < 0.001) significantly and positively influenced purchase intention. However, hedonic value did not significantly influence purchase intention (β = 0.052, p = 0.245). Mediation analysis revealed full mediation of the hedonic value-purchase intention relationship and partial mediation of the utilitarian value-purchase intention relationship by trust.

The findings largely support the hypotheses, except for the non-significant impact of hedonic value on purchase intention. The positive relationship between utilitarian value, information quality, and trust with purchase intention aligns with previous research. The unexpected insignificance of hedonic value suggests that during a health crisis, consumers prioritize functional benefits and trustworthy information over pleasurable aspects when making health-related purchases. This contrasts with findings in contexts without a health crisis, highlighting the influence of situational factors on consumer behavior. The study contributes by demonstrating the significant role of UGC in shaping consumer decisions without direct marketing interventions, highlighting the power of peer-to-peer recommendations in health-related product choices.

This study demonstrates the significant influence of UGC quality, quantity, trust, and utilitarian value on Vietnamese consumers' purchase intention for dietary supplements during and after the COVID-19 pandemic. Hedonic value, however, proved insignificant. The findings offer valuable insights for marketers and managers in the dietary supplement industry. Future research should explore other product categories, broader geographical regions, and additional UGC attributes to enhance the study's generalizability.

The study's limitations include its focus on a specific product category (dietary supplements) and a single geographical area (Vietnam), limiting generalizability. The sample size, while substantial, may not fully represent the entire Vietnamese population. Furthermore, the study only considers a subset of potential UGC factors that could affect purchase intention. Future research could explore a wider range of products, geographical areas, and additional UGC elements.